add docs for get_states and changed empty list declaration
add docs for get_states and changed empty list declaration

abinashpanda/pgmpy,pratyakshs/pgmpy,sandeepkrjha/pgmpy,kislayabhi/pgmpy,kris-singh/pgmpy,sandeepkrjha/pgmpy,vivek425ster/pgmpy,liquidmetal/pgmpy,ankurankan/pgmpy,yashu-seth/pgmpy,khalibartan/pgmpy,pgmpy/pgmpy,khalibartan/pgmpy,liquidmetal/pgmpy,palashahuja/pgmpy,jhonatanoliveira/pgmpy,jhonatanoliveira/pgmpy,yashu-seth/pgmpy,kislayabhi/pgmpy,abinashpanda/pgmpy,palashahuja/pgmpy,anaviltripathi/pgmpy,kris-singh/pgmpy,anaviltripathi/pgmpy,pgmpy/pgmpy,ankurankan/pgmpy,pratyakshs/pgmpy,vivek425ster/pgmpy